1. Field of the Invention
This invention relates to a video signal reproducing apparatus and more particularly to a video signal reproducing apparatus having a memory which is capable of storing a predetermined period portions of the video signal.
2. Description of the Related Art
The conventional reproducing apparatus of the above stated kind include a rotary head type video signal reproducing apparatus (hereinafter referred to as VTR) which is provided with two oppositely arranged magnetic heads having different azimuth angles. The known tracking control methods applicable to the VTR of this type include a method of using pilot signals of four different frequency values. This tracking control method has been employed even in carrying out a special reproducing operation called slow motion reproduction.
FIG. 1 of the accompanying drawings shows in a block diagram the above stated tracking control method of using the pilot signals of four different frequency values as applied to a slow motion reproducing operation. FIGS. 2(a) to 2(d) and 3(a) to 3(d) respectively show, in timing charts, the reproducing operations performed by the conventional VTR at tape travel speeds 1/2 and 1/3 of a normal reproducing tape speed. Of these drawings, FIGS. 2(a) and 3(a) analytically show track patterns representing the tracing state of the head of the apparatus obtained on a magnetic recording tape during the slow reproducing operation. FIGS. 2(b) and 3(b) show the relation of local pilot signals to the fields. FIGS. 2(c) and 3(c) show the voltage wave forms of tracking error signals obtained by the four frequency method. FIGS. 2(d) and 3(d) show the wave forms of sampling pulses produced for the purpose of sampling ATF signals.
Referring to FIG. 1, among the components of a reproduced video signal coming via an input terminal 101, a pilot signal component is alone extracted from the reproduced video signal by a low-pass filter 102 (hereinafter referred to as LPF). The pilot signal component and a local pilot signal which is generated by a local pilot signal generator 103 are subjected to a multiplying operation performed by a multiplier 104. The output of the multiplier 104 is supplied to an ATF signal generating circuit 105. The circuit 105 then produces an analog ATF signal which corresponds to tracking deviation and is obtained in a known manner. The ATF signal thus produced is sampled and held by a sample-and-hold circuit 106 in accordance with sampling pulses coming from a sampling pulse generator 108. The output of the circuit 106 is used as a signal for phase control over a capstan motor system.
In slow motion reproduction, the local pilot signal generator 103 and a sampling pulse generator 108 are controlled by a system controller 107 as described below with reference to FIGS. 2(a) to 2(d) and 3(a) to 3(d):
In each of FIGS. 2(a) and 3(a), the axis of abscissa shows a time base, with one pitch thereof corresponding to one field period. The axis of ordinate shows an extent to which the tape is moved, with one pitch thereof corresponding to one track pitch. Numerals 1, 2, 3 and 4 written in these drawings represent the suffixes of the reference symbols denoting the frequency values of the above stated pilot signals of four different frequency values f1, f2, f3 and f4 which are superimposed on the video signal recorded on the tape.
During ordinary reproduction, tracking control is performed in the following manner: A track A1 in which the pilot signal of the frequency f1 is recorded is first traced by a rotary head. A track B1 in which the pilot signal of the frequency f2 is recorded is next traced. A track A2 in which the pilot signal of the frequency f3 is recorded is then traced. After that, a track B2 in which the pilot signal of the frequency f4 is recorded is traced by the head.
In the 1/2 speed reproduction, the head traces the tape as indicated by thick lines in FIG. 2(a). In this instance, a reproduced signal is obtained and produced from hatched parts between the thick lines. As obvious from FIG. 2(a), the reproduction output becomes zero to generate a noise bar once in every four fields. However, the noise bar can be prevented from appearing on a picture plane by performing the tracking control to have the timing of its occurrence at the end of or beginning of each field as shown in the drawing. In view of this, the conventional apparatus has been arranged to control the relative position of the tape and the head in such a way as to obtain a 1/2 speed tracing state which is as indicated by the thick lines in FIG. 2(a) and is considered to be the optimum condition. To accomplish the tracking control, the local pilot signals which are to be used for the multiplying operation by the multiplier 104 are generated by the local pilot signal generator 103 in rotation of frequency values in such a manner that the local pilot signal of the frequency fl is generated for two field periods, then the local pilot signal of the frequency f2 for next two field periods and so on as shown in FIG. 2(b).
Further, as shown in FIG. 2(d), the system controller 107 produces, during the 1/2 speed reproduction, one sampling pulse for every two field periods at a point of time at which one field is changed over to another. With the ATF signal voltage which is as shown in FIG. 2(c) sampled and held according to such sampling pulses, the ATF signal voltage is sampled and held just at the zero level thereof under the above stated optimum condition. Therefore, tracking can be accomplished in the best state as mentioned above in accordance with the ATF signal.
In the case of the 1/3 speed reproduction which is as shown in FIG. 3(a), the generation of the local pilot signals is also changed from the local pilot signal of one frequency over to the signal of another frequency once for every three fields in a manner similar to the 1/2 speed reproducing operation shown in FIG. 2(a). Then, as obvious from FIG. 3(d), each of the ATF signal sampling pulses is controlled to be produced for every three fields at the middle point of the three-field period, because: The ATF signal is at zero level in the middle part of this period in the best tracking state as shown in FIG. 3(a).
The operating timing of the system controller 107 is determined by a rectangular wave signal of 30 Hz (hereinafter referred to as 30 PG signal) which is in synchronism with the rotation of the rotary head.
As mentioned in the foregoing, practice has been to perform tracking control for slow motion reproduction with an effort to have noise occurring outside of the picture plane. In other words, the point of time at which the reproduction output diminishes to a great extent is arranged to occur at a boundary between one field and another and not to occur in the middle of any of the fields. However, this conventional arrangement has presented a problem in that: It becomes largely impossible to obtain an adequately reproduced picture when the output level for a certain field is somewhat low as a whole, because a sufficiently large dynamic range cannot be obtained for such a field.
Further, it has been attempted to obtain an adequately reproduced slow motion picture by intermittently driving the tape in such a way as to alternate ordinary reproduction and still picture reproduction. However, the arrangement to intermittently drive the tape is not desirable because, compared with driving the tape at a constant speed, such arrangement imposes a larger mechanical load on the apparatus.